When conventional linings are utilized for the protection of the shell of cylindrical grinding mills, the large grinding bodies tend to migrate or segregate to the discharge end of the mill, regardless of the portion of the mill in which they are loaded, while the smaller grinding bodies tend to accumulate at the ore feed end. This migration provides an unfavorable environment for efficient crushing or grinding of the ore due to the lack of large grinding bodies in the zone where the ore enters the mill. It is a further disadvantage with respect to the effective grinding area of the grinding bodies in the discharge zone since the presence of the larger bodies reduces the effective grinding area which is necessary for final finishing.
Thus, the efficiency of the grinding process can be improved if this segregation is reversed, since the breaking or crushing power of the larger bodies would be better employed against the larger ore particles introduced into the feed zone, while the friction provided by the smaller bodies would operate better on the smaller ore particles in the discharge end of the mill.
Various methods have been used to remedy this type of migration. The simplest procedure consists of increasing the thickness of the lining towards the discharge side of the mill by either gradually increasing the thickness of the lining or by using structural supports to increase the lining thickness. Truncated, cone-shaped mills have also been used. These make it possible to achieve a situation whereby the large grinding bodies remain in the loading or front sector of the mill, while the smaller bodies are properly distributed throughout the mill but primarily on the discharge side.
For dry grinding, such as that required by the cement industry, ball mills have been used with sections or compartments that are separated by a grating or grooved partition. These provide individual chambers within the mill to maintain a selective segregation or a desired distribution of the grinding bodies.
The design of the lining itself or of the shell protecting plates or shields which make up the lining can be altered to achieve the desired distribution of grinding bodies. For example, Chilean Pat. No. 29,208 entitled "Armor Plate Lining for Tube or Cylindrical Mills", discloses a lining formed by polygonal sections having rounded corners and rectilinear sides which are formed by shielding ring plates. These sections are oriented in such a manner that the rounded and straight edges of the plates are alternated. A structural supporting system, covered by Chilean Pat. No. 29,030 is used for the assembly of this lining.
By utilizing the above-described method, the distribution of the grinding bodies is achieved by means of a four-pitch screw effect imparted by the individual rings of the plates, because these rings are spaced apart from each other along the length of the mill. The assembly of this system, however, is quite complicated and difficult, and structural supports are required in order to obtain the shape of the polygonal cross section of the lining. Furthermore, since the lining does not form a continuous path along the mill, it does not fully achieve the desired size distribution of the grinding bodies throughout the mill.
Another method of distributing particles by size is found in Chilean Pat. No. 29,519, entitled "Improved System for Concentrating Ores by Means of a Spiral Chute". This patent discloses a vertical, descending spiral chute which separates ore particles of various specific gravities. This is not a rotary device, however, and it separates ore pulp by the movement of the particles. The ore flows through this device by gravity, and the centrifugal force imparted by the rotary path of the particles along the spiral configuration of the chute achieves the desired size distribution. This patent, however, does not disclose how to distribute grinding bodies in a horizontal ball mill.
The applicants have now discovered a novel and unobvious protective plate lining for the shells of cylindrical grinding mills wherein plates having a variable pitch ratio from helical flutings which make it possible to obtain a rapid and stationary distribution of the grinding bodies and ore load throughout the length of the mill. This distribution results because the lining imparts a helical forward motion to the grinding bodies and ore particles in a direct ratio to the mass or weight of these bodies and particles. This effect is the result of the combined action of the spacing of the helical flutings, gravity, the rotation of the mill, and the diametrical pitch of the lining.
This new lining does not require the use of additional supports or support elements between the shell and the plates, nor does it require the installation of gratings or partitions to produce separate grinding compartments. Thus, the new lining can be assembled in a much shorter time than conventional linings. Furthermore, the invention does not appreciably reduce the section or useful volume of the mill, and this allows for the processing of greater tonnages of ore than conventional-type ball mills of the same or similar size.